A touch panel is a device that detects the coordinates of a position pointed to by a pointing object such as a finger or a pen, or detects a pointing action by such a pointing object. Today, touch panels are typically used in combination with a display such as a Liquid Crystal Display (LCD), a Plasma Display Panel (PDP), or an organic EL display.
Outputs from a touch panel are input into a computer, which controls images displayed on the display and controls a device, thereby implementing an easy-to-use human interface. Touch panels are now used in a wide range of everyday-life applications such as video game machines, portable information terminals, ticket-vending machines, automatic teller machines (ATMs), and automobile navigation systems. As computers grow more powerful and network connection environments become widespread, a wider variety of services are provided through electronic devices and the need for displays with a touch panel is increasing.
One type of touch panel is a surface capacitive touch panel. The surface capacitive touch panel includes (i) a resistive sheet and (ii) drive and sensing circuitry which is connected to the resistive sheet, applies an AC voltage (sinusoidal voltage) to the resistive sheet as an excitation, measures a current flowing through the resistive sheet, and outputs the measurement.
Specifically, the surface capacitive touch panel includes a transparent substrate, a transparent resistive sheet formed on the surface of the transparent substrate, and a thin insulating film formed on the top surface of the resistive sheet. The resistive sheet is referred to as the position sensing conductive film. To drive the touch panel of this type, an AC voltage is applied to the four corners of the position sensing conductive film. When the touch panel is touched with a human finger, a pointing stick or the like (hereinafter referred to as the indicator), a capacitor is formed by capacitive coupling between the position sensing conductive film and the indicator. A small current flows to the indicator via the capacitor. The current flows from the corners of the position sensing conductive film to the point touched with the indicator. A signal processing circuit calculates whether or not there is a touch with an indicator and, the coordinate of the position touched with the indicator on the basis of the current detected by the drive and sensing circuitry. Specifically, the signal processing circuit detects the presence or absence of a touch on the basis of the sum of currents from the four corners of the position sensing conductive film. In addition, the coordinates of the touch position is calculated on the basis of the ratio of the currents from the four corners of the position sensing conductive film.
Touch panels based on the surface capacitive operation principle described above are disclosed in Patent Literatures 1 to 5.
A device in Patent Literature 1, which includes a display panel and a touch panel combined and operated together, is equipped with counter-electrode driving means for applying an AC voltage to the touch panel during a non-display period of the display panel and applying the same AC voltage to the counter electrodes of the display panel in order to prevent decrease in the precision of position detection due to a drive signal for the display panel.
Patent Literature 2 discloses a “touch panel device in which when noise is large, the level of AC voltage oscillation is increased whereas when noise is small, the AC voltage oscillation level is decreased and, when there is noise with a specific frequency, switching is made to a different voltage oscillation frequency, thereby ensuring safety to achieve an improved signal-to-noise ratio, high noise robustness, and electrical safety”.
Patent Literature 3 states that “a phase and an AC voltage when a finger has touched the panel are set as a contact vector signal and a scalar quantity calculated from the phase difference between the two signals and amplitudes by using the cosine theorem is set as an AC signal of the actual finger touch, thereby excluding the AC voltage due to a parasitic signal when a finger is not near the resistive sheet or the phase difference between the signals due to a finger of a capacitive grounding human body or a resistive grounding human body from the detection of the touch position”.
Patent Literature 4 discloses that “the operational circuit takes an input of an output from the long sensor line LSLi and an output from the short sensor line SSLi and performs a computation using the difference (Delta) between the outputs and the line capacitance ratio Kc to obtain a signal component S”.
Patent Literature 5 discloses that “the four nodes are labeled with symbols Na, Nb, Nc and Nd. Terminals of current sensing circuits, which will be described later, are connected to the nodes” and states that “single-pole double-throw switches 21a to 21d are connected to the nodes Na to Nd through current sensing circuits 13a to 13d. An AC voltage source 22 is connected to one of the two contacts of each of the single-pole double-throw switches 21a to 21d and a storage capacitance line drive circuit is connected to the other contact (i.e. a node labeled with COM in FIG. 4). The waveform of an AC voltage may be a sinusoidal wave, for example”.
On the other hand, the patent applicant including the present inventor presented several proposals in Patent Document 6 and Non-Patent Document 1 to attain an object of “detecting touching or non-touching, or a touch position with high precision by removing noise whose frequency is the same as a signal frequency or as close to the signal frequency as cannot be resolved by a conventional frequency resolution”.